With those few but powerful words, the father of Thomas and Stuart Kelly has expressed the unbearable sadness he and wife are enduring, yet again, after the death of their second son.

Ralph Kelly posted the crushing message on Facebook on Tuesday night after his son, Stuart, 18, was found dead on Sydney's northern beaches on Monday.

Stuart's death, which police said was not being treated as suspicious, came less than a month after the fourth anniversary of his older brother Thomas' death from a one-punch attack in Kings Cross.

Ralph and his wife, Kathy, have not spoken publicly since Stuart's death, but were inundated with messages of support and condolence online after posting the heartbreaking message on social media.

Stuart graduated last year from The King's School in Parramatta, which has also rallied behind the family. The school's headmaster, Tim Hawkes, released a touching tribute to Stuart, describing his death as a "profound shock".

"To have lost two sons in such circumstances is beyond awful," he said.

"The school has already been in contact with the Kelly family and will be offering them all the support we can.

"As a student, Stuart was greatly respected. He was a school prefect who possessed a quiet integrity that made him enormously effective as a leader.

"The strength and character shown by Stuart when his brother was killed by a one-punch attack in July 2012 was extraordinary. Stuart has since gone on to support the work of the Kelly Foundation and to campaign against alcohol-fuelled violence.

"The death of Stuart Kelly is tragic, and a reminder that life is fragile, loved ones precious and each day a gift."

Following Thomas' death in 2012, his family brought the public's attention to alcohol-related violence in King's Cross.

Their fierce advocacy created the momentum that led to strict mandatory sentencing laws being introduced for violent offences fuelled by alcohol.

They also created the Thomas Kelly Youth Foundation, and the associated Take Kare Safe Spaces program, which sends volunteers onto Sydney's streets to assist young people.

Last year, at a gala event to raise money for the foundation, Stuart addressed an audience of more than 700, including the state's most powerful figures, and revealed the lasting impact his brother's death had on him.

He recalled the moment he and his sister, Madeleine, learnt that Thomas' life support would be switched off.

"I look back at that moment: I was 14 years old, I was told by a stranger that my brother, my best friend, was going to die. Those few words would change my life forever," he told the crowd, including NSW Premier Mike Baird, NSW Police Commissioner Andrew Scipione and lord mayor Clover Moore.

"I'm now 17 - that was three years ago. However, I carry a deep scar that you cannot see. It's always there, it never leaves. It sits below the surface of your skin and surfaces when you least expect it."

❏ Support is available for those who may be distressed by phoning Lifeline 13 11 14; Mensline 1300 789 978; Kids Helpline 1800 551 800; beyondblue 1300 224 636.